Proposals to send back refugees en masse from the European Union to Turkey would breach their right to protection under European and international law, United Nations says.
It added that this move would be equal to blanket return of foreigners to a third country.
"The collective expulsion of foreigners is prohibited under the European Convention of Human Rights," UNHCR's Europe regional director, Vincent Cochetel, said in Geneva on Tuesday.
"An agreement that would be tantamount to a blanket return of any foreigners to a third country, is not consistent with European law, is not consistent with international law."
Turkey had offered to take back refugees and migrants who went into Europe from the country and asked for more money in return.